eaa.iop.orgDOI: 10.1888/0333750888/2356 Active Galaxies: OverviewMatthew Malkan FromEncyclopedia of Astronomy & AstrophysicsP. Murdin © IOP Publishing Ltd 2006 ISBN: 0333750888Downloaded on Thu Mar 02 22:46:55 GMT 2006 [131.215.103.76]Institute of Physics PublishingBristol and PhiladelphiaTerms and ConditionsActive Galaxies: OverviewENCYCLOPEDIA OF ASTRONOMY AND ASTROPHYSICSActive Galaxies: OverviewDefining an ‘ACTIVE GALACTIC NUCLEUS’ (AGN) can bedifficult. The last two words are relatively simple, as anAGN is an energetic event which is never found anywherein the universe except within the central parsecs of agalaxy. This is not surprising, because any dynamicalfriction should soon forceany very massive compact objectto settle down into the center of the galactic gravitationalpotential.AGALACTIC NUCLEUS is said to be ‘active’ if it generates a‘substantial’ (or detectable) amount of energyby processesdifferent from those found anywhere else in galaxies. Thatis, the energy of the ‘activity’ is not produced by evolutionof stars and interstellar matter. This definition is cleanbecause it is theoretical. The actual observational taskof identifying AGN can be messy, especially when thenonstellar activity is weak compared with the backgroundof normal stellar processes, including birth and death.Classifying AGN meaningfully is harder. Understandingtheir physical nature is harder still.The problems are further complicated because thenonstellar activity releases energy in many forms, andat photon wavelengths covering nearly the entire elec-tromagnetic spectrum. Since observing instruments andmethods differ markedly from one waveband to an-other, many overlapping wavelength-specific classifica-tion schemes and models have been proposed.Not surprisingly, astronomers became aware of themore spectacular and luminous examples of AGN first.These nuclei were sufficiently luminous that they couldoutshine the entire galaxy in which they resided. Theywerealso very distant, and it was often not possible, beforevery sensitive imaging had detected the surroundinggalaxy, to distinguish their optical appearance from thatof a star. This led to their designation as ‘quasistellar’objects. Some of the first of these were also powerful radiosources, and the term was soon abbreviated to ‘quasar’,which now generally refers to all high-luminosity AGN(seeQUASISTELLAR OBJECTS: OVERVIEW). Their unique naturebecame more obvious when studying their spectra, inalmost any wavelength region.These unique properties have now been found in ac-tive nuclei spanning an extraordinary range in luminos-ity1. There are only surprisingly subtle spectroscopic dif-ferences between a quasar with a luminosity of 1048erg s−1and what had previously been identified as a ‘Seyfertgalaxy nucleus’ of luminosity 1042erg s−1. The evidenceis overwhelming that these two astronomical categoriesmust have a single common physical explanation. Thustoday, hardly any distinction is made between ‘quasar’and ‘Seyfert 1 nucleus’, as their main difference is (some-what arbitrarily) in their luminosity. It follows that thisoverview of AGN should be supplemented by reading theaccompanying chapters on quasars in this Encyclopedia.1The vast range of AGN luminosities, much larger than what isseen in stars for example, is one of their outstanding features.Understanding The Central Engine in AGNThere are several key ideas which are being used toorganize and make sense of our large and growingobservational information about AGN.Unified ModelsSince the earliest AGN studies, it was recognized thatthey can show strong observational differences in spiteof many other similarities. Two characteristics seem tohave a sufficiently bimodal distribution to justify usingthem to divide AGN into two pairs of classes: strongradio emission (which distinguishes radio-loud (RL) fromradio-weak (RW)AGN) and broadpermitted line emission(which distinguishes type 1 Seyfert nuclei from type 2). Amore extensive discussion of these ideas is given inACTIVEGALAXIES: UNIFIED MODELand ACTIVE GALAXIES: VARIABILITY.The majority of quasars are only moderately strongerradio sources than normal galaxies. (They are sometimesreferred to as radio-quiet (RQ) AGN, although strictlyspeaking they do emit some radio flux so that RW is a moreaccurate description.) An important minority of quasarsare well known for the beams of high-energyparticles theyaccelerate. The radio emission from these jets is nearlyalways bipolar—a key indicator that the central engine canhave axial symmetry. In many models, this jet–lobe axisis the spin axis of the central massive black hole. In themostdramatic subsample ofRLAGN, the‘blazars’ (looselynamed after the prototype object BL Lacertae), the highlypolarized, violently variable, steep red continuum wasidentified as nonthermal synchrotron emission. Blazaris an informal term used to encompass the prototypicalnonthermal BL Lacertae objects (with very weak emissionlines) as well as the optically violently variable quasars.Interferometric maps showed that the core-dominatedradio emission generally had a one-sided jet, which oftenappears to be expanding at apparent transverse speedsseveral times larger than c. The most extreme of theseobjects appear to emit a substantial fraction of their energyin the gamma rays, and possibly even up to TeV energies.All of these remarkable (and manifestly nonstellar) blazarproperties were soon understood as the result of viewing arelativisticjet (of electronsand either protons or positrons),which is emerging from the active nucleus close to our lineof sight. This nearly ‘pole-on’ viewing angle producesseveral relativistic effects such as Doppler boosting ofthe ‘beamed’ synchrotron emission and a compressionof apparent timescales which accounts for ‘superluminalmotion’ without violating special relativity. The one-sided nature of the innermost jets is usually attributed to‘Doppler favoritism’. If there is currently an outflow in theopposite direction (which is predicted by some theoreticalmodels, but not yet confirmed), it is invisible becauseDoppler de-boosting makes it thousands of times fainter.An immediate consequence of the beaming explana-tion of core-dominated RL AGN is that more AGN withradio jets must be pointing away from Earth. These mustbe detectable by their isotropic emission (lines or extendedradio lobes), as either lobe-dominated RLAGN, or perhapsCopyright ©